...what I don't understand is how it works with the Premium? Do I still have to buy both the template and the Premium service? And can I still personalise the template even if it's ready made (Premium offers the possibility to customise it)?

Thanks - I just got started with WordPress and would appreciate help on this!

The premium upgrade still means you are hosted here on WordPress.com. If you need that specific theme, then you'll need to use WordPress.ORG. Here are details about the differences:http://support.wordpress.com/com-vs-org/

Note that none of the plans allow you to run advertising or to upload your own themes or plugins. WordPress.com blogs cannot be equipped for eCommerce transactions. If you need a site that requires a custom theme or plugins — e.g., if you want to be able to sell things directly from your site, or if you want to install discussion forums — a self-hosted WordPress site might be right for you.

WordPress.org offers free software that you can install on a web server. You can upload and install themes and plugins, run ads, and edit the database. Learn about the differences here. http://support.wordpress.com/com-vs-org/

Upgrades are also available for individual purchase on a per blog basis. With the exception of the Guided Transfer, all upgrades are per year and per blog. You will find the relevant links to available upgrades here. http://en.support.wordpress.com/upgrades/

You can do that with the domain mapping upgrade.http://en.support.wordpress.com/domains/
Keep in mind that you are still hosted here for free on WordPress.com; it's kind of like a street sign which points your readers to where your site actually is: Your real address might be 555 main street, but people visiting you see it as "My Great Magazine."

Yes, those are the themes we can use here! The only themes we can use here. Some are free, some you buy. And if you decide later to move to WordPress.ORG, then you'll have to purchase a different version because our wp.com themes won't work on wp.org.

@vildamagazine
One last thing:
You may find that you don't need all the features in the premium bundle useful: are you going to have videos? The 3GB of storage is enough space for thousands and thousands of properly compressed photos, so you may not need that, especially as you are just starting out.
You can pick and choose which upgrades you need; the bundle saves money but only if it includes things you'll use.

Also keep in mind that many themes have quite a bit of built in custom features, so you can change the colors and looks without any extra cost.

<Opinion> -

many people get worried about the looks of their blog and spend tons of time changing the looks of the site and not blogging - I don't think anyone visits a site "because it looks way cool" - they go for the content - but you can mess up a theme and the way a blog works and drive visitors away